UWB radiofrequency signals are signals whose bandwidth (defined at −10 dB of the maximum of the spectral power density) is greater than 500 MHz or than 20% of the central frequency. In particular, certain so-called impulse systems (Impulse Radio—UWB) provide for the transmission of coded and/or modulated trains of brief pulses (of the order of an ns).
UWB technology is on the way to establishing itself as favored physical layer of short and medium range wireless digital communication networks, such as wireless personal area networks (WPANs) and sensor networks or WSNs (for Wireless Sensor Networks).
The present invention proposes that means for aiding the characterization of the environment in which the emitter/receiver nodes of a UWB wireless communication network are situated be incorporated into the network, either for the purposes of optimizing the UWB communication network, or for other purposes related for example to the utilization of the knowledge of the configuration of the environment (for example the geometric configuration of the walls, their constituent materials) with the aim of providing integrated services within a communication network.
In the first case, where it is desired to aid the optimization of a UWB network, it is understood that it is particularly indicated to obtain an item of information about physical characteristics, notably geometric characteristics, of the environment (presence of obstacles to transmission which attenuate the signals, presence of reflecting objects which disturb transmission by creating multi-paths). This for example enables a network to better adapt to its environment by taking into account the real environment whose characteristics it has itself logged; the adaptation can be done for example by using, on reception, filters suitably matched to the transmission channel or by acting on the routing of the communications in the network.
In the second case, it is desired to acquire information about the environment with the aim of providing the users of the network with an additional service. UWB radio emitters/receivers forming part of a UWB communication network then determine a geometric configuration (presence of walls, of partitions, position, etc.) and electromagnetic configuration (that is to say the nature of these walls) of the environment; this may be with the aim of determining whether the environment is in a potentially dangerous state or with the aim of detecting a modification of the configuration (for example an intrusion); this may also be with the aim of informing the other, potentially distant, nodes of the network about the current state of the environment. These nodes can thereafter process the information dependent on the topology of the environment with other data measured at various points of the sensor network (for example, temperature, pressure, etc.).
The expression “characterization of the environment” is understood to mean the determination of configurations or changes of configuration of the physical elements which are present in the environment of the emitter-receiver nodes of the communication network.
In the prior art, it has already been proposed that radio mapping be carried out inside buildings (“indoor mapping”) by means of emitters-receivers dedicated to this mapping but not by using an in situ communications network having an aim other than this mapping. This mapping is based on an analysis of the echoes of pulses sent by an emitter. The instants of arrival of the various multi-path echoes resolved by the receiver are associated with the multiple reflections which may have occurred between emission and reception. These reflections occur on walls whose relative position and whose orientation with respect to an emitter/receiver pair (potentially co-located) can then be determined by means of simple geometric relations. The article by W. Guo and N. P. Filer “2D Indoor Mapping and Location Sensing Using an Impulse Radio Network” in IEEE Seminar on Ultra Wideband Systems, technologies and Applications, 2006 pp. 211-215 London, April 2006, or also the article by W. Guo, N. P. Filer and R. Zetik, entitled “Indoor Mapping and Positioning Using Impulse Radios” in IEEE Position Location and Navigation Symposium, pp. 153-163, April 2006, propose this type of scheme.
Another technique has also been proposed: it consists in using a multiplicity of receivers dedicated to characterization, at known positions, so as to determine the presence and the position, in the environment, of objects liable to reflect the waves. See for example R. Zetik, J. Sachs, R. Thöma, “Imaging of Propagation Environment by UWB Channel Sounding”, COST 273 TD(05) 058, Bologna, January 2005.
Use has also been made of environmental signature techniques in which there is a receiver: in a training phase, profiles of power received as a function of time are collected during the emission of pulses by an emitter, doing so by placing receivers in several regions in space. Mean profiles are stored for each region in space, and, during the use of a receiver, a profile of power received is compared with the previously stored profiles, so as to deduce therefrom the region in space in which the receiver is probably situated. See for example the article by F. Althaus, F. Troesch and A. Wittneben, “UWB Geo-Regioning in Rich Multipath Environment, IEEE 2005.
Sophisticated schemes have also been proposed, which use neural networks to recognize patterns of multi-paths resolved on reception, with the aim of allowing positioning of the receiver in a pre-established environment. See for example C. Nerguizian, C. Despins, S. Affes, “Geolocation in Mines with an Impulse Response Fingerprinting Technique and Neural Networks”, IEEE VTC04-Fall, Los Angeles, vol. 5 p 3589-3594, September 2004.
All these techniques are specific detection techniques, using emitters or receivers dedicated to the detection of objects in the environment.
Other examples of radiofrequency systems for environment reconstruction have also been proposed, for example in patent application EP 2003/721985 or in patent application US 2006 405167A. These systems do not utilize the characteristic of deformation of a pulse subsequent to radiation or to capture by a UWB antenna.
In patent application US 2006 405167A “Method and apparatus for utilizing RF signals to create a site specific representation of an environment”, there is described a scheme comprising the estimation of the distance traveled by the wave between the emitter and the receiver in the course of successive emissions during which an emission antenna is rotated 5° by 5° so as to scan an observation space. The algorithm described in this prior application makes provision to previously perform an estimation of the distance traveled (for example on the basis of a measurement of the outward-return flight time of the signal transmitted), and then a link budget taking account of the gain (in power) of the antennas and of the propagation losses (path loss), and finally to associate the additional energy losses observed on reception (that is to say the additional losses with respect to the initial budget) with the type of channel, or indeed with the electromagnetic interaction phenomena. This algorithm does not take into account the deformation of pulses to deduce environmental characteristics therefrom.
Finally, in patent application US 2003 714046A, the determination of reception angles relies on the use of a network of n antennas, which are capable of associating the signal received with one of the 2n sectors delimited by the antennas themselves. The determination of deformations of impulse waveforms is not suggested.
The prior art techniques allow environmental characterization (sometimes coarse), but they use emitters and receivers dedicated to this characterization and not a multi-node communications network dedicated mainly to the communication of information between nodes. Moreover, none of the schemes cited above relies on the deformations undergone by the waveforms associated with the paths resolved on reception.